It has been postulated for many years that the cardiac atria serve as sensors that are important in detecting changes in extracellular fluid volume (Gauer et al., Physiol, Rev. 43: 423, 1963). Such a receptor function for the cardiac atria is known in the case of vasopressin, the hypothalamic hormone important in regulating the osmotic concentration of the body fluids.
The postulated existance of a substance which would enhance urinary sodium excretion, and hence be involved in regulation of extracellular fluid volume, was demonstrated recently. De Bold et al., Life Sci. 28: 89, 1981, injected a partially purified extract of cardiac atria of rats into other anesthetized rats and observed a large increase in urine flow and in urinary sodium excretion. This relatively crude extract possessed the appropriate characteristics of an endogenous natriuretic substance.
In addition to its potent diuretic and natriuretic effects, properties that make the material especially appropriate to exert a major effect on body fluid volume regulation, it was also discovered that these extracts of cardiac atria have potent smooth muscle relaxant activity (Currie et al., Science 221: 71, 1983). Such action implies a potential direct role in regulating blood pressure as well as a role in regulating extracellular fluid volume.
Because of the immediately recognized importance of this discovery for understanding the regulation of body fluid volume and blood pressure and the obvious therapeutic potential of such a natural substance in the treatment of congestive heart failure and hypertension, numerous laboratories set about to isolate, characterize and chemically identify the active substance(s) in the cardiac atrial extracts. The active substance(s) in cardiac atria was called atrial natriuretic factor or ANF but has been referred to also as cardionatrin (de Bold et al., Life Sci. 33: 297-302, 1983) and atriopeptin (Currie et al., Science 111: 67, 1984). Atrial natriuretic factor was shown to be a family of peptides all of which have a common amino acid sequence but differ in length by the presence or absence of 1-8 amino acids on the amino or carboxyl termini.
Peptide chemists quickly produced completely synthetic material that mimicked the biological activity of the family of peptides that have been isolated from the cardiac atria.
The biological activity of ANF indicates utility in congestive heart failure where standard therapy utilizes potent diuretics in combination with peripheral vasodilating drugs. Atrial natriuretic factor combines both of these actions in one molecule which is produced naturally within the body. It is possible that the salt and water retention associated with congestive heart failure is a result of inadequeate production of ANF. If such is true, administration of ANF would allow for replacement of adequate quantities of the material.
A second major disease for which the biological activity of ANF indicates utility is essential hypertension. Standard therapy for hypertension utilizes diuretic and peripheral vasodilating drugs. Atrial natriuretic factor incorporates both of these characteristics. A specific use also may be found in the acute treatment of hypertensive crisis such as malignant hypertension where the powerful vasodilating effect of ANF would be paramount.
In addition to these two very broad categories of therapeutic utility, it is possible that those diseases which are characterized by decreases in renal function may benefit because of the favorable action of ANF on renal hemodynamics, especially enhancement of medullary blood flow.
U.S. Pat. No. 4,609,725 by Brady et al. teaches that synthetic fragments of ANF can be labelled at carboxyl terminus in the tyrosine residue by electrophillic labelling with iodine and that these radiolabelled derivatives are useful for studying the in vivo and in vitro metabolism of ANF.